SERVER-14835 - fix fassert 16143 on PPC64 LE systems in LogFile::synchronousAppend()

new version, using ioctl(_fd, BLKBSZGET) and fixed predefined alignment for PPC64.
We don't really need to check for _direct here. Just assert on reinterpret_cast<ssize_t>( buf ) % _blkSize == 0.